Wireless communication systems provide voice and data communications between one or more fixed transceivers, known as base stations or Node Bs, and a plurality of mobile transceivers, or User Equipment (UE), across an air interface. The voice or data information (herein collectively referred to as data) are encoded, modulated onto a radio frequency (RF) carrier. The modulated RF signal is amplified, transmitted across the air interface, received, demodulated, and the demodulated bits are decoded and further processed to recover the data. The physical channel, which includes circuits in the transmitter, the air interface, and circuits in the receiver, introduces impairments (interference and noise), making the received signal differ, often significantly, from the transmitted signal. The inverse of the extent to which the channel impairs the transmitted signal is referred to as channel quality. A common metric of channel quality is the Signal to Interference and Noise Ratio (SINR); another is the block error rate (BLER).
One method of improving spectral efficiency for a given channel quality is link adaptation, also known as adaptive coding and modulation (ACM). In one form of link adaptation, a transmission format (TF), which specifies the modulation type, forward error correction (FEC) coding rate, number of transmit antennas to employ (i.e., space-time code), number of spatial multiplexing streams, and other parameters, is adaptively selected from a fixed number of possibilities, in response to dynamic measurements of the channel quality. To enable assessment of the channel quality, known data patterns, referred to as reference signals or pilots, are transmitted across the air interface. By comparing received reference signals to their known value, a receiver can assess and quantify the impairment characteristics, and report the channel quality.
Several types of reference signals are used for channel characterization. For example, in LTE uplink, two types of reference signals are employed. A sounding reference signal (SRS) is used to facilitate frequency dependent scheduling. A demodulation reference signal (DMRS) facilitates coherent demodulation. Particularly in a rapidly changing channel, sending more reference signals increases channel characterization accuracy and hence improves link adaptation. However, reference signals decrease spectral efficiency, as they consume air interface bandwidth yet do not transmit user data. Additionally, in practice, impairments arising from imperfections in the transmitter and receiver circuits are difficult to accurately estimate using reference signals.